The invention relates to an instrument switch having a rocker switch seated in an insulating housing which, with a working end that dips into the housing interior, moves a contact bridge between the closed position of the contact and open position of the contact, depending on the pivot position, and having a contact spring that is connected electrically in series with the contact bridge and can be thermally triggered and hence acts as overcurrent protection.
The external electrical connections of this instrument switch are connected to one another by means of two switching elements disposed inside the insulating housing of the instrument switch and connected in series. The one switching element is a contact bridge that can move between the closed position of its contact and the open position of its contact. The contact bridge is acted upon by a manually-operated rocker switch pivotably disposed on the insulation housing, and can therefore interrupt the circuit. In this way the instrument switch can be manually turned on and off.
The second switching element acts as protection against overcurrent. It is configured as a contact spring that can be triggered thermally and, when activated, likewise interrupts the circuit. By nature the thermally-related switching movement of the contact spring to interrupt the circuit is very slow. However, the interruption of the circuit requires rapid switching movements of a switching element, particularly with stronger currents. For this purpose the overcurrent protection of an instrument switch is realized in U.S. Pat. No. 4,528,538 and U.S. Pat. No. 5,079,530 by a so-called starting mechanism. Starting mechanisms are known from W. Krause, Konstruktions-Elemente der Feinmechanik [Construction Elements of Precision Mechanics], pp. 521 et seq., Second Edition, Munich-Vienna, Carl-Hanser-Verlag 1993, ISBN 3-446-16530-4. The starting mechanisms used in U.S. Pat. No. 4,528,538 and U.S. Pat. No. 5,079,530 each include an energy store in the form of a spring. The heat energy generated due to overcurrent is stored in the spring as mechanical energy. The stored energy is released at a specific point in time that is dependent upon the amount and duration of the overcurrent. Because of this, slow switching movements are converted into snap-like switching movements.
After the thermal triggering element has been activated and has interrupted the circuit, it cools again and attempts to return to its initial position. However, the circuit would then be closed again. In many cases it is desirable that the overcurrent protection not automatically re-close the circuit after the thermal triggering element has cooled. In U.S. Pat. No. 4,528,538 this automatic re-closing of the circuit is prevented by a blocking element coupled with the rocker switch. Unfortunately, the blocking element requires additional componentry for the instrument switch. The spatial arrangement of the rocker switch and the contact spring also requires a correspondingly space-consuming design of the blocking element. Moreover, the forces acting upon the blocking element require it to have stable seating. This counteracts simple handling of the rocker switch with little use of force. The wearing effect of the forces acting upon the blocking element can cause the blocking element to operate imprecisely. The reliable operation of the instrument switch can thus no longer be assured. In U.S. Pat. No. 5,079,530 the contact spring itself has a very complicated design so that it can prevent automatic re-closing of the circuit by the contact spring after a completed overcurrent trip. The complicated design makes the contact spring very susceptible to interference, which likewise impairs the functioning reliability of the instrument switch. Furthermore, as a result of the complicated design of the contact spring, only small force transmissions take place between the individual components. In its function as a switching element in a closed circuit, the contact spring therefore also only generates a low contact force. Also, the structural design of the rocker switch is made complicated so that a coupling of the rocker switch with the contact spring can be achieved.